Docs: Add Sigstore to python.org/downloads

[hugovk]

Summary

Brought up at https://github.com/python/cpython/issues/127602#issuecomment-2517304604.

If you go to a release page (for example, https://www.python.org/downloads/release/python-3131/) and click "Sigstore" in the table header, it'll take you to some docs at https://www.python.org/downloads/metadata/sigstore/

But we don't have anything at https://www.python.org/downloads/

Let's add something brief at https://www.python.org/downloads/ and link to the more detailed guide at https://www.python.org/downloads/metadata/sigstore/

cc @sethmlarson